1. Field of the Invention
The present invention relates to the field of wireless communications.
2. Description of Related Art
In wireless communication networks based on spread spectrum technology, such as a Code Division Multiple Access (CDMA) network, a plurality of mobile subscriber terminals (xe2x80x9cmobilesxe2x80x9d) share the same radio frequency (RF) bandwidth, and are separated by employing different Walsh codes or other orthogonal functions. As compared to communication systems which create multiple channels from a single RF band by assigning different time slots to users, i.e., Time Division Multiple Access (TDMA), or subdividing an RF band into a plurality of sub-bands, i.e., Frequency Division Multiple Access (FDMA), using orthogonal code sequences to form separate channels enables a CDMA system to exhibit xe2x80x9csoftxe2x80x9d network capacity. In other words, the number of mobiles which can share a given RF bandwidth at one time is not fixed, and instead is typically limited only by the degradation of service quality caused by interference from other users of the same and adjacent cells/sectors. The resulting tradeoff between network capacity and service quality in a CDMA system is typically resolved by reverse link (mobile to base station) power control techniques which adaptively set mobile transmit power to the minimum level needed to maintain adequate performance.
Despite the use of reverse link power control techniques to reduce co-channel interference and increase capacity, overload may occur in network cells/sectors when the number of mobiles being served exceeds the maximum number at which target call quality (typically represented as the ratio of energy per bit, Eb, to noise and interference, No, in a given bandwidth) can be maintained, for example when a large number of mobiles attempt to communicate with a single base station at once. One previously implemented technique for avoiding overload relies on a call admission/blocking scheme to guarantee adequate communication quality by blocking service to additional subscribers when load levels exceed a certain threshold. Such call admission schemes, however, may result in unacceptable service outages.
The present invention is a system and a method which scales base station transmit signals in a wireless communication network in response to high load levels, thereby affecting handoff control values measured at served mobiles to xe2x80x9cpushxe2x80x9d mobiles to adjacent cells/sectors and avoid overload conditions. In one implementation, a base station overload controller scales the amplitude of aggregate forward link (base station to mobile) transmission signals as a function of the difference between aggregate transmit signal magnitudes and a threshold level. By scaling aggregate base station transmit signals, which include control signal components (e.g., a pilot signal component in a CDMA system), handoff control values, including receive signal strength, bit/frame error rates, and signal-to-noise ratio, measured at mobiles within the network service area are affected. Depending on the location of mobiles and the degree to which the aggregate base station transmit signals are scaled, a percentage of served mobiles, particularly those at cell/sector boundaries, will request handoff to an adjacent cell/sector. As the load level increases relative to the threshold level, the degree of scaling likewise increases, thereby more significantly affecting handoff control values measured at mobiles within the network service area, and causing an increased number of handoffs to balance load between a plurality of cells/sectors. Thus, the present invention increases network capacity and prevents overload without relying solely on a call admission scheme.
In one embodiment, the present invention is an aggregate overload controller which samples and sums aggregate in-phase (I) channel and quadrature (Q) channel transmit signal magnitudes over a load measurement period to obtain a load measurement value, and outputs a scaling coefficient as a function of the difference between the load measurement and a threshold. The aggregate overload controller initially sets the scaling coefficient to 1, and maintains the scaling coefficient at 1 as long as the load measurement value remains below the threshold. When the load measurement first exceeds the threshold, the scaling coefficient from the preceding load measurement period (i.e., 1) is decreased by an offset value which is calculated as a function of the difference between the load measurement value and the threshold. In one implementation, the updated scaling coefficient is calculated as:
SM=min{1, SMxe2x88x921+xcexc(Ethxe2x88x92EM)},xe2x80x83xe2x80x83(1)
where SMxe2x88x921 is the scaling coefficient from the previous load measurement period, Eth is the threshold, EM is the load measurement for the current load measurement period, and xcexc is a constant. The constant xcexc may be set to a small value, e.g., 0.01, to prevent substantial fluctuations in the scaling coefficient SM, and thereby avoid network instability.
I- and Q-channel multipliers multiply the scaling coefficient SM received from the aggregate overload controller by aggregate I- and Q-channel transmit signals received from a baseband processor. The resulting scaled I- and Q-channel transmit signals are received by an RF processor, which performs digital-to-analog conversion, low-pass filtering, modulates the scaled I- and Q-channel transmit signals onto separate RF carriers, combines the modulated I- and Q-channel carriers, and outputs the combined RF transmit signal to base station antenna for transmission.